Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
1999-11-05
2001-07-10
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
C560S261000
Reexamination Certificate
active
06258978
ABSTRACT:
The present invention relates to a process for the production of vinyl acetate.
Vinyl acetate may be produced by the acetoxylation of ethylene. In a typical vinyl acetate production process, ethylene, acetic acid and oxygen are introduced into a reactor via an inlet. The reactants are contacted with a palladium-containing catalyst and react to produce an outlet stream which is removed from the reactor and cooled. Vinyl acetate, water and the unreacted acetic acid in the outlet stream are condensed and separated for further purification. The remaining gaseous components of the outlet stream (e.g. ethylene) are compressed and recycled.
The rate of acetoxylation increases as the concentration of oxygen in the reactor is increased. However, the amount of oxygen that may be introduced into the reactor is limited by the flammability limit of the reactant mixture. The flammability limit is defined as the highest concentration of oxygen in a mixture that will fail to sustain combustion. If the oxygen concentration exceeds this flammability limit, a fire or explosion could result.
Various steps have been taken to minimise the risk of such a fire or explosion. For example, in the fixed-bed reactor of EP 0 845 453, the concentration of oxygen in the inlet gas composition is closely monitored and maintained at or near a threshold value. The mathematical approximations used to define this threshold value are described in EP 0 845 453 which is incorporated herein by reference. When the inlet oxygen concentration exceeds this threshold value, a shut-down signal is activated, and the reaction is quenched by shutting off the ingress of fresh oxygen into the reactor.
A problem with this arrangement is that the oxygen concentration in the reactor is limited by the flammability limit of the feed mixture in the inlet, rather than the flammability limit of the reaction mixture in the reactor itself. In general, a higher concentration of oxygen may be tolerated in the latter and, accordingly, the shut-down signal may be activated too soon before the concentration of oxygen in the reactor reaches an optimum value.
The problem may be avoided in a fluid bed reactor by introducing fresh oxygen via a separate inlet as described in U.S. Pat. No. 5 550 281. This arrangement, however, is unsuitable for fixed bed systems.
We have now developed a process for producing vinyl acetate in which the concentration of oxygen in the reactor is not limited by the amount of fresh oxygen that is introduced into the reactor via an inlet. This process is applicable to both fixed bed and fluid bed reactors.
According to the present invention, there is provided a process for the production of vinyl acetate, said process comprising the steps of:
(a) introducing ethylene, acetic acid and an oxygen-containing gas into a reactor,
(b) reacting said ethylene, acetic acid, and oxygen-containing gas in the presence of an acetoxylation catalyst in said reactor to produce a process stream,
(c) removing said process stream from the reactor as an outlet stream, and maintaining the oxygen concentration of said outlet stream at or near its flammability limit.
The present invention has the advantage that by maintaining the concentration of oxygen in the outlet stream at or near its flammability limit, an increase in the productivity and selectivity of the vinyl acetate production process is observed.
As explained above, the flammability limit of a mixture is defined as the highest concentration of oxygen in the mixture that will fail to sustain combustion. This limit may be expressed as a function of pressure, temperature and the composition of the mixture, as described by the empirical equations disclosed in EP 0 845 453.
In this application, the term “outlet stream” is taken to encompass the initial stream emerging directly from the reactor and any stream subsequently derived from the initial stream, except for the stream entering the reactor at its inlet. For example, after leaving the reactor, the outlet stream may be returned to the reactor via a recycle loop involving one or more processing stages. In one processing stage, the outlet stream emerging from the reactor may be introduced into a separation unit where liquid components of the outlet stream such as vinyl acetate, water and/or unreacted acetic acid are removed. The separation unit may take the form of one or more distillation columns.
In a further processing stage, some or all of the outlet stream leaving the separation unit may be introduced into a compressor. Some of the outlet stream leaving the separation unit may then be introduced into a carbon dioxide removal unit. Here, some or all of the carbon dioxide in the outlet stream is removed. The outlet stream is then re-directed into the compressor to complete the recycle loop.
The composition of the outlet stream varies at different points along the recycle loop. For example, the composition of the outlet stream emerging from the reactor may be different to the composition of the outlet stream emerging from each of the various processing stages of the recycle loop.
Together with changes in temperature and pressure, these differences in composition may cause the flammability limits of the outlet stream to vary at different points along the recycle loop. For example, in one embodiment, the flammability limit of the outlet stream between the reactor and separator unit is different to the flammability limit of the outlet stream between the separator unit and the compressor. The flammability limit of the outlet stream may also change after compression and, subsequently, after carbon dioxide has been removed. To avoid the risk of fire/explosion, the oxygen concentration of the outlet stream should be maintained at or below the flammability limit at all points along the recycle loop.
The flammability limit in the outlet stream may be up to and including 10 vol. % oxygen, for example 7 vol. % oxygen. Suitably, the oxygen concentration in the outlet stream is maintained at or below 10 vol. % oxygen.
The process of the present invention may further comprise the step of shutting down the reactor in the event that the concentration of oxygen in the outlet stream exceeds or is likely to exceed its flammability limit. To determine whether shut-down is necessary, the oxygen concentration of the outlet stream is monitored, for example, by computer at various points along the recycle loop. For example, in one embodiment, the oxygen concentration of the outlet stream is monitored at four stages along the recycle loop. Firstly, as the outlet stream emerges from the reactor; secondly, as it emerges from the separation unit; thirdly, as it emerges from the compressor and fourthly, as it emerges from the carbon dioxide removal unit. When the oxygen concentration at any one of these stages exceeds a threshold value defined by the flammability limit, the shut-down signal is activated.
In an alternative embodiment, the oxygen concentration of the outlet stream is monitored at a single stage or “trip point”. When the oxygen concentration at this trip point exceeds a threshold value defined by the flammability limit, the shut-down signal is activated. In theory, the trip point may be defined as the point along the recycle loop at which the outlet stream is at or closest to its flammability limit. In practice, however, fluctuations in temperature, pressure and outlet stream composition may cause the outlet stream to exceed its flammability limit other than at the trip point. This has to be taken into account when calculating the threshold oxygen concentration at the trip point. The threshold value may be defined by a flammability equation such as described in EP-A-0845453 (incorporated herein by reference) with allowance for errors (for example 95% confidence limit), for residence time in the sampling, for trip system response time, for equipment accuracy and for natural variations in the plant operation.
The threshold value may suitable be set at or below 10 vol. % oxygen. For example, with a flammability limit of 7 vol. % the threshold value may
Kitchen Simon James
Thomson Alasdair Iain
Williams Bruce Leo
BP Chemicals Limited
Deemie Robert W.
Killos Paul J.
Nixon & Vanderhye
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